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Nervous System: Part II
How A Neuron Works
Essential Knowledge Statement
3.E.2 Continued
Animals have nervous systems
that detect external and internal
signals, transmit and integrate
information, and produce
responses
2
Identify the Numbered Structures
Describe a Resting Potential:
• What is the charge inside the neuron at rest?
• Why is the cell negative inside and positive
outside? (be specific)
5
Source of Charge Differences:
6
Action Potential
• Action potentials propagate impulses along
neurons.
– Membranes of neurons are polarized by the
establishment of electrical potentials across the
membranes.
– In response to a stimulus, Na+ and K+ gated
channels sequentially open and cause the
membrane to become locally depolarized.
– Na+/K+ pumps, powered by ATP, work to maintain
membrane potential.
7
Label the graph of the action potential as we go
through the next several slides.
8
Generation of Action Potentials:
A Closer Look
• An action potential can be considered
as a series of stages
• At resting potential
1. Most voltage-gated sodium (Na+)
channels are closed; most of the
voltage-gated potassium (K+) channels
are also closed
Key
Na
K
Membrane potential
(mV)
50
0
Threshold
50
100
OUTSIDE OF CELL
INSIDE OF CELL
Inactivation loop
1
Resting state
Sodium
channel
Potassium
channel
1
Resting potential
Time
• When an action potential is generated
2. Voltage-gated Na+ channels open first and
Na+ flows into the cell
3. During the rising phase, the threshold is
crossed, and the membrane potential
increases to and past zero
Key
Na
K
Membrane potential
(mV)
50
0
50
2
Depolarization
OUTSIDE OF CELL
INSIDE OF CELL
Inactivation loop
1
Resting state
100
Sodium
channel
Potassium
channel
Threshold
2
1
Resting potential
Time
• When an action potential is generated
2. Voltage-gated Na+ channels open first and
Na+ flows into the cell
3. During the rising phase, the threshold is
crossed, and the membrane potential
increases to and past zero
4. During the falling phase, voltage-gated
Na+ channels become inactivated; voltagegated K+ channels open, and K+ flows out
of the cell
Key
Na
K
50
Rising phase of the action potential
Membrane potential
(mV)
3
Action
potential
50
2
Depolarization
OUTSIDE OF CELL
INSIDE OF CELL
Inactivation loop
1
Resting state
100
Sodium
channel
Potassium
channel
3
0
Threshold
2
1
Resting potential
Time
Key
Na
K
50
Rising phase of the action potential
Membrane potential
(mV)
3
Depolarization
OUTSIDE OF CELL
INSIDE OF CELL
Inactivation loop
1
Resting state
Potassium
channel
3
0
100
Sodium
channel
Falling phase of the action potential
Action
potential
50
2
4
Threshold
2
1
Resting potential
Time
4
5. During the undershoot, membrane permeability to
K+ is at first higher than at rest, then voltage-gated
K+ channels close and resting potential is restored
Key
Na
K
50
Rising phase of the action potential
Membrane potential
(mV)
3
Depolarization
OUTSIDE OF CELL
3
0
100
Sodium
channel
Falling phase of the action potential
Action
potential
50
2
4
Threshold
2
4
1
5
Resting potential
Time
Potassium
channel
INSIDE OF CELL
Inactivation loop
1
Resting state
5
Undershoot
1
Figure 48.11a
50
Membrane potential
(mV)
Action
potential
3
0
2
50
4
Threshold
1
5
Resting potential
100
Time
1
Refractory Period
• During the
refractory period
after an action
potential, a second
action potential
cannot be initiated
• The refractory
period is a result
of a temporary
inactivation of the
Na+ channels
Conduction of Action Potentials
• At the site where the
action potential is
generated, usually the
axon hillock, an electrical
current depolarizes the
neighboring region of the
axon membrane
• Action potentials travel in
only one direction:
toward the synaptic
terminals
• Inactivated Na+
channels behind
the zone of
depolarization
prevent the
action potential
from traveling
backwards
Axon
Action
potential
1
Na
Plasma
membrane
Cytosol
Axon
Plasma
membrane
Action
potential
1
Na
K
2
Cytosol
Action
potential
Na
K
Axon
Plasma
membrane
Action
potential
1
Na
K
2
Cytosol
Action
potential
Na
K
K
3
Action
potential
Na
K
Sequence the following in order of occurrence
• Depolarization
• Resting state
• Repolarization
• Hyperpolarization
Sequenced in order of occurrence
• Resting state
• Depolarization
• Hyperpolarization
• Repolarization
• Resting state
Evolutionary Adaptations
of Axon Structure
• The speed of an action potential increases
with the axon’s diameter
• In vertebrates, axons are insulated by a
myelin sheath, which causes an action
potential’s speed to increase
• Myelin sheaths are made by glia—
oligodendrocytes in the CNS and Schwann
cells in the PNS
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
0.1 m
Next time we will explore what
happens when the impulse
reaches the end of the axon.
29
Created by:
Debra Richards
Coordinator of Secondary Science Programs
Bryan ISD
Bryan, TX